1. Field of the Invention
This invention relates to friction materials such as clutch linings, brake pads, linings, blocks and the like. More particularly this invention relates to asbestos-free formulations of friction materials which incorporate a palygorskite clay exemplified by attapulgite.
2. Prior Art
Friction material compositions currently used in clutch and brake linings of vehicles, motors and other machinery must be capable of withstanding severe operating temperatures and high pressures experienced during repeated applications. In addition the materials must have surface coefficient of friction suitable to the requirements of the application and must have wear properties to balance the cost of their use and avoid undesirable side effects such as glazing, squeak, fade and grooving in the mating surfaces. Compositions in general use comprise a thermoset binder exemplified by phenolic molding compositions, or novolak resins, a fibrous reinforcement to prevent physical degradation and deterioration in performance, various fillers and other additives including wear modifiers, lubricants, friction modifiers and the like. The fibrous reinforcement in widest use is asbestos. The asbestos provides the phenolic molding composition with superior mechanical and thermal properties but constitutes a health hazard to those exposed to it.
Compositions containing other fibrous reinforcement as an asbestos replacement have been investigated, but such materials have had limited commercial acceptance. When formulating asbestos-free friction materials, end-use properties such as friction coefficient and wear, heat resistance and mechanical strength should be equivalent to asbestos-containing compositions. In addition, processing properties such as moldability and shrinkage should also be equivalent.
In U.S. Pat. No. 3,835,118 to Rhee et al. the use of steel fibers in combination with sponge iron filler to replace asbestos is disclosed. In U.S. Pat. No. 3,922,241 to Barker et al the use of a novolak resin with aluminum silicate (not further defined), talc and cellulose fiber in place of asbestos is disclosed. Other asbestos-free formulations include those disclosed in U.S. Pat. No. 4,119,591 to Aldrich, wherein steel and cellulose fiber are used and carbon fiber, mineral fiber and glass fiber are disclosed; U.S. Pat. No. 4,197,223 to Bartram, in which glass fiber, mineral fiber and cellulose are disclosed as an asbestos replacement, and U.S. Pat. No. 4,226,759 to Chester, in which steel fiber alone is disclosed. In order to meet performance and processing criteria, such as those mentioned above, substituting another material for asbestos is generally not sufficient in itself; rather the overall formulation often requires change or adjustment to meet the demands the product must satisfy.
Thus an object of this invention is to provide a friction material composition formulated without asbestos but having performance and processing characteristics equivalent to existing products made with asbestos.